I was thinking about this .. in her conversation with police, Weier says that many people do not believe that Slenderman is real and she wants to prove the skeptics wrong .. so I figure there is some kind of online debate going on about this somewhere, here is an answer from Yahoo Answers about it ..
Is Slender Man real or fake?
I saw a documentary online and it has a decalssified tape about a man's daughter disappearing, and the cop who's interviewing him gets taken. The whole hornet something rather series shows some pretty creepy and realistic stuff too. And after all, it is possible to have such a creature. Theories and explanations please?
This is the doco:
THE FOLLOWER: THE SLENDER MAN DOCUMENTARY - YouTube
Here is another one:
Slender-Man Info and Documentary - YouTube
Leah answered 3 months ago
He is real if you believe in him, and your mind will make it real, or if you think hes fake, then hes not real then. I'm sure you'll be fine if you don't think about Slender. There had been people who had seen slender, but disappeared and nowhere to be found, and by that, I mean the ppl were missing. Don't believe in him. Please.
https://au.answers.yahoo.com/question/index?qid=20110616172538AA6GrSv
So from that does that mean, that if these girls DECIDED to make him real, they could do so by committing this crime and THEN be able to find and enter his mansion and live with him .. does that make sense?
Proof that he is fake:
Proof that Slender Man is fake - YouTube (bear with me on all this) ..
The Slenderman is real proof:
Slender-Man Sightings Scary Pictures ! - YouTube
From Creepypasta:
http://creepypasta.wikia.com/wiki/The_Slender_Man
Breakdown from DeviantArt on how old woodcuts were altered to show that Slenderman is not real:
http://annoyedslendermanfan.deviantart.com/art/Regarding-Slenderman-Throughout-The-Ages-347375664
The point I am making is that everywhere you look at this story online you find exasperated people telling the occasional poster that they need to see this proof, stop posting 'in character' (if they are) and understand that Slenderman is NOT REAL but was made up, they then give links to show the origin of the tale, only a few posts later to be confronted by another Slenderman believer and having to go through the entire process again.
I think the girls were likely part of a forum where they were in heated discussions with Slenderman skeptics .. since in the mythos of Slenderman if you believe in him he becomes real, and your mind will make him real, then in order to win their argument, they may have literally made themselves believe in order to manifest him, as it were.
Here are the early video's that supported the Slenderman myth:
Introduction - YouTube
Entry #1 - YouTube
Every great horror story is written as if real, otherwise there would be no fun in it, but I wonder .. these girls were likely arguing with people a lot older than they are, who may have wanted to mess with them a little, because as 12 year old kids they would have come across as impossibly naive and of course another poster would not know their actual age.
Just a thought. Perhaps someone online even suggested they try to become proxies to find out?